RF communication systems employing angle modulation (e.g., frequency modulation (FM) or phase modulation (PM)) for transmission of data, have historically been extremely susceptible to the deleterious effects of co-channel interference. Co-channel interference is caused by the transmission of a signal from a nearby source on the communications channel of concern. As is known, conventional FM receivers suffer from the basic inability to distinguish a desired signal from an undesired signal transmitted by an interfering source, especially when the transmissions are on the same communications channel (i.e., frequency).
In order to overcome this shortcoming and guarantee adequate levels of system performance, communication systems employing FM receivers often require that all such receivers must maintain a predetermined desired signal (i.e., carrier) to interference (C/I) ratio. Typical cellular communication systems, such as the Advanced Mobile Phone Service (AMPS) or Narrow band Advanced Mobile Phone Service (NAMPS) cellular systems deployed by Motorola Inc. require a C/I ratio of at least 18 decibels (dB). While 18 dB of separation between the desired carrier and an interference source is adequate to protect the system from co-channel interference, it nonetheless fails to maximize spectral efficiency, especially when one of the known frequency reuse schemes is also employed by the system in question.
It would be extremely advantageous therefore to provide a radio receiver architecture which operates at a lower C/I separation than is taught by the prior art, in the effort to increase overall spectral efficiency without adversely impacting system performance. Of note, additional information on the AMPS and NAMPS systems has in the past been available by contacting Motorola Inc. at 1303 East Algonquin Road, Schaumburg, Ill. 60196.